Electrical devices such as power resistors and the like generate significant heat during operation and it is usual to provide such devices with a heat transfer medium for transferring heat from the device to a suitable heat sink such a metal plate or other body of heat conducting material.
A power resistor is described in U.S. Pat. No. 5,355,281 in which a heat generating electrically conductive element is secured to one side of a bonded ceramic-copper laminate plate. The heat-generating element is enclosed within a resistor housing by attachment of the heat conducting plate to an open end of the housing. The laminated plate comprises an intermediate layer of nickel-plated copper sandwiched between first and second alumina (aluminum oxide) ceramic layers. The heat-generating element is secured to the alumina substrate on one side of the plate while the ceramic substrate on the other side of the plate is nickel-plated and is located on the exterior of the assembled device. Internally, the element is electrically connected to a terminal provided on the exterior of the housing. Typically, in devices of this type, the interior of the housing is filled with a so-called “potting compound” of silicon resin insulating material which is mixed under vacuum conditions to eliminate voids in the insulation so that partial discharge of the high voltage resistor element is minimized during operation.
The service life of high voltage electrical devices is usually limited by breakdown of the insulation, as measured by partial discharge. Partial discharge increases over time as insulation deteriorates due to the growth of voids in the body of the insulation material due to spark erosion. Spark erosion of the insulation occurs due to variations in the electrical field strength at voids in the body of the insulation material and at the edges of the insulation where divergence of the electrical field is greatest. Although partial discharge can be measured relatively easy, it is extremely difficult to predict or observe where it occurs.
It is therefore desirable to improve the quality of the insulation and hence partial discharge characteristics and service life of high voltage electrical devices such as power resistors of the aforementioned type.